In recent years, wearable devices have been developed progressively with the spread of Internet of Things (IoT). Representative examples thereof include a watch and glasses that can be connected with internet. Wearable devices that can always monitor physical conditions are also necessary in a medical field and a sports field, and are expected to be a growth field in the future.
In the medical field, wearable devices have been investigated to monitor organic conditions by sensing a weak current such as an electrocardiogram measurement, which detects heart beats by electric signals. The electrocardiogram is measured by fitting a body with electrodes on which electro-conductive paste is applied, and this measurement is performed only once in a short period of time. On the other hand, the aim of development of the foregoing medical wearable device is to develop devices that monitor health conditions continuously for several weeks. Accordingly, bio-electrodes used for a medical wearable device have to keep the electric conductivity unchanged and not to cause skin allergies even when being used for a long time. In addition to these, it is desirable that the bio-electrode is light in weight and can be manufactured at low cost.
Medical wearable devices include a type in which the device is attached to a body and a type in which the device is incorporated into clothes. As the type in which the device is attached to a body, it has been proposed a bio-electrode using water soluble gel containing water and electrolyte, which are materials of the foregoing electro-conductive paste (Patent Document 1). The water soluble gel contains sodium, potassium, or calcium as the electrolyte in a water soluble polymer for retaining water, and converts changes of ion concentration from skin into electricity. On the other hand, as the type in which the device is incorporated into clothes, it has been proposed a means to use cloth in which an electro-conductive polymer such as poly-3,4-ethylenedioxythiophene-polystyrenesulfonate (PEDOT-PSS) or silver paste is incorporated into the fibers for electrodes (Patent Document 2).
When using the foregoing water soluble gel containing water and electrolyte, however, the electric conductivity is lost as the water is lost due to drying. On the other hand, some people can cause skin allergies by the use of metal with high ionization tendency such as copper. The use of an electro-conductive polymer such as PEDOT-PSS also has a risk of skin allergies due to the strong acidity of the electro-conductive polymer.
As the electrode material, it has been investigated to use metal nanowire, carbon black, and carbon nanotube since they have excellent electric conductivity (Patent Documents 3, 4, and 5). The metal nanowire can conduct electricity in a small loading amount since the wires are brought into contact with each other in high probability. The metal nanowire, however, can cause skin allergies since they are thin material with sharp tips. The carbon nanotube also has stimuli to a living body by the same reason. The carbon black has some irritativeness to skin, although the toxicity is lower than the carbon nanotube. As described above, the biocompatibility is sometimes worsened due to the shape and irritativeness of a material, even though the material itself does not cause an allergic reaction. Accordingly, it has been difficult to achieve both the electric conductivity and the biocompatibility.
As a means for solving these problems, it has been investigated to use electro-conductive metal particles as an electrode material. Among metals, noble metals such as gold, platinum, and silver, which have lowest ionization tendencies, are hard to cause skin allergies. Accordingly, it is possible to achieve both the electric conductivity and the biocompatibility by using these noble metal particles. When mixing these noble metal particles into a resin, however, electricity is not conducted unless the particles are brought into contact with each other in the resin, which is an insulator. In order to bring the particles into contact with each other, the noble metal particles have to be loaded in a volume ratio of 70% or more. As described above, when using metal particles, it is necessary to load a large amount of expensive noble metal particles, and accordingly, the production cost becomes very high and the weight increases, thereby making it impossible to achieve weight reduction, which is necessary for wearable devices.
When the bio-electrode is away from skin, it becomes impossible to obtain information from the body. Just the change of contact area fluctuates the quantity of electricity to be conducted, thereby fluctuating the baseline of an electrocardiogram (electric signals). Accordingly, the bio-electrode have to be in contact with skin continually without changing the contact area in order to obtain stable electric signals from a body. For that purpose, the bio-electrode preferably has tackiness. It also needs elasticity and flexibility to cope with expansion and contraction as well as change of bending of skin.